A parson is a parish priest, and with this Parson we get an exemplary one. Unlike the Friar or the Monk, who fail to practice what they preach, the Parson lives the Gospel he teaches by being holy and virtuous in all things, giving to the poor while he himself lives a life of poverty, and visiting his widely-spaced parishioners, rain or shine. The Parson is explicit about the motivation for his behavior: he must serve as a good example to his parishioners, for "if gold rust, what shal iren do?" (General Prologue 502). That is, if a religious figure can't live a holy life, how can he expect a layperson to do so?
In the Parson's portrait we see a lot of pastoral imagery, or language about sheep and shepherds. The Parson sees his parishioners as his sheep, and says that he cannot leave them stuck in the mud. He reinforces his reasons for living a holy life by saying that it wouldn't be right for a flock of white sheep to be watched by a "shiten" (dirty) shepherd, someone bespattered with sin. From this language we get the impression that the Parson truly views himself as the caretaker of Christian souls. He takes his responsibility extremely seriously.
Many parish priests at this time period chose to take a "benefice," or position far away from their parish, in which their only job might be to say mass for one departed rich person once a day. This was a way for a priest to make much more money, but it required him to live far away from his parishioners. By rejecting this option, the Parson shows that he is willing to sacrifice his own comfort to do his job as a shepherd of souls.
Comparing the Parson with characters like the Friar, Monk, Prioress, Pardoner, and Summoner makes those characters look even more selfish and sinful. The Parson's presence on the pilgrimage is necessary for precisely this reason: without him, we have no idea of what a religious figure ought to be, while, with him, we have an almost Christ-like point of comparison.